This invention relates to self-baking electrodes for use in electric arc furnaces and a method of making the same.
One type of conventional self-baking electrodes comprises a vertically disposed cylindrical casing which extends downwardly through an opening in the roof of an electric arc furnace. The upper end of the casing is open to permit the insertion of a carbonaceous paste-like material which first melts and then cures to a solid state as it passes downwardly through the casing as a result of heat which is conducted upwardly from the cured portion of the electrode extending below the lower end of the casing. Such paste may be made, for example, by calcining anthracite or petroleum or asphalt cokes which is then mixed with a bonding material such as pitch or tar.
In one type of prior art consumable electrode, such as that shown in U.S. Pat. No. 3,715,439, metallic fins are fixed to the internal surface of the casing and extend inwardly toward the center of the electrode to act as a reinforcement and to promote curing by conducting heat to the interior portions of the electrode. This patent also discloses the use of a log core in the electrode to reduce the content of paste required.
Another type of prior art consumable electrode was formed by feeding a carbonaceous electrode forming material into the open upper end of a metallic casing. Electrical current was delivered to the electrode by means of a contact assembly commonly having contact plates and a surrounding pressure ring. A variable pressurizing means, such as a hydraulic cylinder, adjusted the pressure between the contact plates and the casing. A metallic structure was located at the axis of the electrode to which it became bonded as the lower end of the electrode was baked. The metallic structure was generally X-shaped in crosssection and was formed by elongate ladder-like sections. In order to minimize contamination of the material being treated by the metal in the electrode casing, the lower portion of the electrode was lowered by means of the metallic structure through the casing while sufficient pressure was maintained between the contact plates and the casing to prevent the casing from slipping through. Periodically, as the lower edge of the casing was consumed through arcing and oxidation, the entire electrode was lowered while the pressure between the contact plates and the casing was reduced to allow the total composit electrode to slip. As the electrode moved downwardly to replace the portion that was consumed at its tip, the electrode was heated by the high temperature existing within the furnace and by electrode current flowing into the electrode via the contact plates.
In order to control the electrical conditions within the furnace, it is necessary to maintain the lower end of the electrode at a controlled distance from the molten bath. As the lower end of the electrode is consumed, therefore, it is necessary to move the electrode downwardly into the furnace. This necessitates that additional sections of the metallic casing and the fins be attached to the upper end of the electrode and further quantities of electrode forming paste be added. Along with the consumption of the electrode, the metallic casing and fins are also consumed. This results in iron being added to the furnace melt thereby adversely affecting products being produced in the furnace, such as silicon, in which high purity is a requisite.
One method of resolving the problem of iron introduction in the operation of self baking electric arc furnace electrodes is disclosed in U.S. Pat. No. 3,819,841. In particular, this patent discloses a self baking electrode wherein metallic fins are not employed. Rather, heated air is delivered to a space between the surface of the metal casing and a mantle which is spaced from and surrounds the casing. This provides sufficient heat for curing the electrode without the necessity for heat conducting fins. Insulation on the inner surface of the mantle assists baking by minimizing heat loss. In addition, the metallic casing is maintained in a stationary position and feeding of the electrode toward the furnace is accomplished by forcing the cured portion of the electrode outwardly from the lower end of the casing. This substantially minimized the introduction of iron into the melt.
It has been found that the baked portion of the electrode lies below the 250.degree. C. isotherm which begins in the region of the lower end of the electrode contact plates and extends upwardly and inwardly toward the center of the furnace. As long as the paste is baked as it leaves the contact plates, there is no problem with rupture of the electrode being initiated at the outer surface regardless of the strength at that location. However, electrode failure becomes a concern when there is a high rate of electrode consumption, such as in the production of silicon. Should the electrode be consumed at a rate such that the baking zone extends below the level of the contact plates, a zone of weakness is created which can result in electrode breakage. This problem is further complicated when a wooden core or log is provided within the electrode as discussed in U.S. Pat. Nos. 3,715,439 and 3,819,841. The wooden core being less heat conductive than the electrode material tends to retard rather than promote the rate of electrode baking.